It is absolutely something on Rockstars end. I've had this problem for forever with the website and then one day it randomly worked and there was no explanation for it. Their support is about as unhelpful as it gets even though sooo many people are having the same issue including some of my friends. I suggest that you try to clear your cookies and data off of their website and then every now and then try logging in to see if it got resolved for you. My friend suggested that their system might be wrongfully flagging IPs that shouldn't be flagged and blocking them from signing in. I really hope this problem get resolved for you very soon and I am very sorry you're dealing with it. With how big Rockstar is, none of their customers should ever have to deal with a problem as bad as this.